This invention generally relates to liquid crystal display devices and more particularly to liquid crystal display devices that have at least two bistable states which are metastable in nature.
Currently, liquid crystal display devices are generally employed as a visual display output in office equipment, e.g., personal computers, notebooks, fax machines and other such display apparatus. These devices generally employ either a twisted nematic (TN) liquid crystal medium or a supertwisted nematic (STN) liquid crystal medium. Examples of such display mediums are described in the article of M. Schadt and W. Helfrich entitled, "Voltage-Dependent Optical Activity of A Twisted Nematic Liquid Crystal", Applied Physics Letters, Vol. 18(4), pp. 127-128 (1971) and in the article of T. J. Scheffer and J. Nehring "A New, Highly Multiplexable Liquid Crystal Display", Applied Physics Letters, Vol. 45(10), pp. 1021-1023 (Nov. 15, 1984). These mediums do not have a lasting stable memory effect. Therefore, these mediums need to be actively driven, e.g., by a matrix driving method using voltage averaging or by an active matrix driving method in which a transistor or other active element is provided at each picture element or pixel.
Other liquid crystal (LC) systems are being currently researched and developed in addition to the foregoing, although they have not reached a reliable product stage. For example, high speed addressing technologies utilizing voltage averaging are disclosed in Japanese Laid-Open Patent 59-219720 and Japanese Laid-Open Patent Publication 60-196728. LC systems utilizing bistable switching properties are disclosed in Japanese Patent Laid-Open Publication 1-51818, which is equivalent to U.S. Pat. No. 4,239,345; Japanese Laid-Open Publication 3-26368; and Japanese Laid-Open Patent 59-58420.
However, these disclosed LC systems inherently have several problems. When a twisted nematic liquid crystal is driven by voltage averaging, the ratio of the voltage V.sub.ON, for selecting the ON condition, to the voltage V.sub.OFF, for selecting the OFF condition, is given by the following equation: EQU V.sub.ON /V.sub.OFF =((N.sup.1/2 +1)/(N.sup.1/2 -1)).sup.1/2,
wherein N is the number of scan lines.
As can be seen from this equation, as the V.sub.ON /V.sub.OFF ratio approaches 1 as N becomes large, the contrast ratio falls. Considering the electrooptical characteristics and voltage waveforms of current liquid crystal mediums, N is limited to approximately 500 lines. Therefore, it is impossible to use this LC system to realize LC display devices for workstations, LC projection systems and the like requiring high precision displays. Also, the display characteristic is significantly dependent on the viewing angle, and, further, the switching time from the ON condition to the OFF condition is comparatively long.
The LC system disclosed in Japanese Laid-Open Patent 60-196728 is aimed at shortening the required switching time. By the disclosed addressing means in this publication, the switching from the ON condition to the OFF condition can be advanced by providing for opposite signs proscribed to the pretilt angle, .theta..sub.1, on the lower substrate, i.e., the angle formed between the molecular director of the liquid crystal molecules in contact with the alignment layer formed on the substrate surface, and the pretilt angle, .theta..sub.2, on the upper substrate. Also, Japanese Laid-Open Patent 59-219720 is aimed at stabilizing the operating condition by adding a chiral component to the liquid crystal medium. However, since these LC systems do not have bistable capabilities and are operated by the voltage averaging method, they are not suitable for use in high precision and high resolution display devices.
Existing operating LC systems having bistable characteristics or plural stable states are suited to high precision displays using many scan lines when switching between states can be selectively performed with suitably applied voltage waveforms. The term, "bistable", as used herein includes metastable states as well as any other pseudo states that can be maintained a period of time greater than the time required to change the entire display screen state. However, each of these bistable systems have their own inherent problems and drawbacks. For example, U.S. Pat. No. 4,239,345 discloses bistable LC systems wherein data that has been written into the display can be retained in its written state for a long period of time without need for refresh or the continued maintenance of an ON condition or applied power for retention of its written state. However, since switching between the two stable states is basically performed by rapid cut off of the applied voltage and its gradual decline over approximately one second, this technology is not suited for use in conjunction with a simple matrix driving method and its writing speed is also extremely slow. Actually, U.S. Pat. No. 4,239,345 only describes a switching principle without disclosing a method for accomplishing a simple matrix drive system.
In Japanese Laid-Open Patent Publication 59-58420, the write voltage can be selected by controlling the applied voltage. However, in order to clear the display of written data, the liquid crystal medium must be heated until the medium takes on an isotropic phase. Furthermore, a significantly high voltage is required for writing into the display cells.
Also, in these LC systems having bistable characteristics or plural stable states, the applied stable state employed for display of data is frequently not sufficiently stable with respect to the applied energy. Therefore, the molecular orientation state of the liquid crystal medium in the condition wherein the power to the display is cut off changes to a molecular orientation state of the lowest energy level. In cases wherein an initial state is involved, such as, in the case of the present invention, and where the molecular orientation states are several and are different, there is a problem in display write scanning, i.e., writing to the display. When write scanning is performed on the initial orientation state during display power up, three orientation states exist together, even through for a short period of time, which degrades display quality.
It is, therefore, an object of this invention to provide a high definition, high precision liquid crystal display device having bistable capabilities that can be driven by a simple matrix display method of operation.